The present invention relates to a fluid pumping system for an automatic transmission.
A power transmission has a fluid pumping system, driven by an engine to deliver fluid to a transmission hydraulic system. The hydraulic system distributes pressurized oil for such transmission operations as shifting gears, torque conversion, lubrication, and cooling. During vehicle acceleration from a low engine speed, relatively high pressure is needed to quickly transfer torque through the gears. Therefore the pump of the fluid pumping system draws substantial energy from the engine. The single pump must be sized to accommodate such maximum flow and pressure requirements of the transmission. This also means that the pump produces excess capacity during lower flow demand operating conditions such as when the vehicle is cruising at higher engine speed. The excess fluid is exhausted through a regulator valve and is therefore a power loss in the system. One proposed solution to improve efficiency is to include a second pump in the pumping system, operating to supplement a lower capacity main pump only during maximum flow demand.
The transmission hydraulic circuit to which the fluid pumping system supplies pressurized fluid may include both a low pressure circuit and a separate high pressure circuit each having individual pressure and flow requirements. The low pressure circuit may deliver lubrication throughout the transmission, whereas the high pressure circuit may deliver high pressure fluid to quickly activate friction devices such as clutches and bands in the gearing mechanism. Since the pump scavenges energy directly from the engine, it is desireable to reduce the pump energy draw when not needed to meet the requirements of the automatic transmission thereby reducing fuel consumption.
The present invention is for a fluid pumping system for an automatic transmission, and particularly to support two independent hydraulic circuits within the transmission hydraulic system. The pressure delivered to each of the independent hydraulic circuits is individually regulated to that particular circuit""s requirements. The fluid pumping system provides this functionality while improving fuel efficiency.
The primary pump is in continuous flow communication with the main hydraulic circuit and selectively connected to the secondary circuit through a main circuit regulator. The secondary pump is connected to the secondary circuit via a one-way check ball valve. A secondary circuit regulator regulates pressure delivered to the secondary circuit.
As excess pump flow is developed from the primary pump during certain operating conditions, the main circuit regulator may open to route excess primary pump flow directly to the secondary hydraulic circuit. If the flow provided by the primary pump is sufficient to meet the demands of the secondary circuit, then the secondary circuit regulator shifts to re-circulate flow from the secondary pump back to the sump and not to the secondary circuit. Here the secondary pump essentially freewheels as it operates against zero pressure, substantially reducing the horsepower draw, when the engine is operating at high speeds sufficient for the primary pump to meet the flow demands of both circuits or when there is low flow requirements. This mode of operation improves vehicle fuel economy. Further, if the primary pump is delivering more flow than is needed by the secondary circuit, the excess flow from the primary pump may be diverted by the secondary circuit regulator back to the sump.